This invention relates to surface wave devices, and is particularly concerned with spatial harmonic transducers for surface wave devices. The term xe2x80x9csurface wavexe2x80x9d is used herein to embrace surface acoustic waves (SAWs), including leaky SAWs, surface skimming bulk waves, and similar acoustic waves, and is abbreviated to SAW below. A spatial harmonic transducer is an interdigital transducer (IDT) in which a number of electrodes have a predetermined polarity sequence which is repeated over a plurality of periods along the length of the transducer.
As is well known, SAW devices provide significant advantages, such as low cost, small size, and desirable filter characteristics, in various filtering and delay applications, especially in wireless communications systems. However, the limitations of photolithographic techniques which are used for manufacturing SAW devices have made it difficult to provide SAW devices with the desired characteristics for operation at very high frequencies, for example exceeding 1 GHz and especially at frequencies of about 2 GHz for current wireless communications systems.
For example, it would be desirable to be able to provide low loss FIR (finite impulse response) SAW device filters for operation as channel select filters at a first IF (intermediate frequency) of about 2 GHz in a 28 GHz wireless communications systems. A SAW device using spatial harmonic transducers can be provided to operate at 2 GHz with excellent selectivity, but such transducers are bidirectional and provide high loss (e.g.  greater than 20 dB). SAW resonator structures can provide IIR (infinite impulse response) filters for operation at this frequency with very low loss (e.g.  less than 5 dB), but have other characteristics such as shape factor, bandwidth, passband amplitude ripple, and group delay which make them ill suited for channel select filters. SPUDT (single phase unidirectional transducer) SAW devices can provide excellent characteristics at lower frequencies, but require a higher resolution to produce their narrower electrodes, and manufacture of SPUDT SAW devices for operation at frequencies of about 2 GHz has not yet been practical.
In a wireless communications system operating at a radio frequency (RF) of about 2 GHz, it is known to provide feed forward error correction, for which a delay of about 130 ns is required. This delay is typically provided by a coaxial cable with a length of about 40 meters. It would be very desirable, especially in terms of cost and size, to replace this cable with a SAW delay device operating at this RF and providing a wide fractional bandwidth, low insertion loss, and linear phase throughout its pass band.
Accordingly, a need exists to provide improved SAW devices for operation at particularly high frequencies.
According to one aspect, this invention provides a SAW (surface wave) device comprising an inter-digital transducer on a substrate of piezoelectric material, the transducer comprising a plurality of groups of electrodes, the electrodes of each group having a predetermined polarity sequence, the groups being arranged along a length of the transducer to operate coherently for SAWs at a signal frequency of the transducer, the signal frequency having a spatial harmonic relationship to a frequency corresponding to a pitch of said electrodes, wherein the transducer further comprises at least one reflector for reflecting SAWs at the signal frequency, the reflector being positioned relative to said groups of electrodes to provide constructive interference of SAWs at the signal frequency in one direction along the length of the transducer and to provide destructive interference of SAWs at the signal frequency in an opposite direction.
Preferably the reflector comprises reflector electrodes having a pitch determined by a SAW wavelength at the signal frequency, and the electrodes of said groups have a pitch determined by a SAW wavelength at a lower frequency to which the signal frequency is spatially harmonically related. Desirably each electrode has a width of at least about xcex0/4 where xcex0 is the SAW wavelength at the signal frequency.
In an embodiment of the invention described below, the groups of electrodes are all similar to one another and have the same polarity sequence. Each group of electrodes comprises five electrodes with a polarity sequence ++xe2x88x92+xe2x88x92, or eight electrodes with a polarity sequence ++xe2x88x92+xe2x88x92xe2x88x92+xe2x88x92. The SAW device can advantageously comprise two of said transducers arranged for propagation of SAWs between them, said one direction, for constructive interference of SAWs at the signal frequency, for each transducer being towards the respective other transducer. In this case, for each transducer, the groups of electrodes can be all similar to one another with the same polarity sequence, and the two transducers can have different groups of electrodes and polarity sequences for the same signal frequency.
Another aspect of the invention provides a SAW (surface wave) device interdigital transducer comprising a plurality of groups each of q consecutive electrodes having a predetermined polarity sequence, each group having a length rxcex0 along the transducer, for transducing SAWs with a wavelength xcex0 at a frequency f0, where q is a positive integer greater than 2 and r is a positive integer less than q and not equal to q/2, the groups being arranged along the length of the transducer to operate coherently for SAWs at the frequency f0, the transducer further comprising at least one reflector for reflecting SAWs at the frequency f0, the reflector being positioned relative to said groups to provide constructive interference of SAWs at the frequency f0 in one direction along the length of the transducer and to provide destructive interference of SAWs at the frequency f0 in an opposite direction, the reflector comprising one or more reflector electrodes, each reflector electrode and each space between reflector electrodes having a width of at least about xcex0/4.
Preferably the reflector is arranged between two of said groups of electrodes and has a length equal to an integer number of wavelengths xcex0; in particular the reflector may have a length of rxcex0 or an integer multiple thereof. In one embodiment described below, q=5 and r=3; in another embodiment, q=8 and r=5.
The invention also provides a SAW device comprising two transducers each as recited above on a substrate of piezoelectric material, the transducers being arranged for propagation between them of SAWs at the frequency f0, each transducer having said one direction, for constructive interference of SAWs at the frequency f0, towards the respective other transducer.
In one form of such a SAW device described below, for each transducer, all groups of electrodes are similar to one another and have the same polarity sequence. In this form of the SAW device, preferably the groups of electrodes and polarity sequences of one of the two transducers are different from the groups of electrodes and polarity sequences of the other of the two transducers. For example, for one of the transducers q=5 and r=3, and for the other of the transducers q=8 and r=5.
The invention further provides a SAW device comprising a spatial harmonic transducer having a preferential direction for propagation of SAWs at a spatial harmonic signal frequency of the transducer, the transducer comprising a plurality of groups of electrodes having a predetermined polarity sequence for transducing SAWs at said frequency, and at least one reflector for reflecting SAWs at said frequency, the reflector being positioned relative to said groups of electrodes to provide relative constructive interference of SAWs at said frequency in one direction along the length of the transducer and to provide relative destructive interference of SAWs at said frequency in an opposite direction, the reflector comprising one or more reflector electrodes, each reflector electrode and each space between reflector electrodes having a width of at least about one quarter of a SAW wavelength at said frequency.